Method of shot peening processing and article processed thereby, and peening material and use thereof

ABSTRACT

This invention provides a method for the treatment of shot peening by which a high residual compressive-stress can be given, while having the surface roughness of the treated article be low even when a metal substance with a high hardness is used. That is to say, this invention is related to a method for the treatment of shot peening wherein a peening material having a Vickers hardness (Hv) in the range of 900-1100 and a Young&#39;s modulus of 200,000 MPa or less, is used, and wherein a treated article is obtained by such a method that has a maximum residual compressive-stress of 1600 MPa or more and a surface roughness of 5 microns or less.

FIELD OF THE INVENTION

[0001] This invention relates to a method for the treatment of shotpeening and a treated article obtained by the process.

DESCRIPTION OF THE RELATED ART

[0002] It is conventionally known that to lengthen the life cycle of ametal product, a metal substance is to be treated by shot peening. Avariety of materials are known as the peening materials used for theshot peening; however, among them a peening material made of cast steelhas been conventionally and widely used. This peening material made ofcast steel can give a high residual compressive-stress to the metalsubstance by increasing the speed of peening. However, there had been aproblem in that due to the increased speed of peening, the surface ofthe metal substance might be damaged.

[0003] When the surface hardness of the metal substance is increased byheat treatment, it is difficult to give it a high residualcompressive-stress by increasing the speed of peening to the-maximum.This is because the peening material is crushed. For this reason, thecases have been increasing where a peening material made of cementedcarbide is used. (For example, see the Gazette of Japanese PatentEarly-publication No. Hei. 08-323626.) When the peening material made ofcemented carbide is used, a metal substance having a high surfacehardness can be provided with a high residual compressive-stress. But,still in this case there is a problem in that the surface of the metalsubstance can be sometimes damaged.

[0004] A so-called treatment of double peening, which at first treats ametal substance with the peening material having a larger diameter, thentreats it with the peening material having a smaller diameter, is acommonly used technique. But, there has been a problem in that pluralpeening equipment is indispensable for it. Proposed is a so-called mixedpeening, which peens a metal substance by using a peening materialhaving a larger diameter and a peening material having a smallerdiameter. However, this technique has a variety of unsolved problems,such as the mixing ratio of the two peening materials, the control ofthe grain size, and the like, and it has not led to a practical use.

SUMMARY OF THE INVENTION

[0005] This invention has been accomplished to solve the above problems.The invention provides a method for the treatment of shot peening thatprovides a metal substance having a relatively high hardness as well asa low hardness, and with a high residual compressive-stress, and readilyprovides a smaller surface roughness of a treated article. The inventionalso provides a treated article obtained by this method. The inventionfurther provides a peening material and a method of its use.

[0006] The invention aims to solve the problems in the conventionaltreatment of shot peening from the aspect of the peening material. Theinventions comprise the following inventions, 1-8.

[0007] 1) One of the methods for the treatment of shot peening of theinvention is characterized in that a peening material having a Vickershardness (Hv) in the range of 900-1100 and a Young's modulus of 200,000MPa or less (more preferably a Young's modulus in the range of50,000-150,000 MPa) is used.

[0008] 2) One of the methods for the treatment of shot peening in theinvention is characterized in that an iron-based amorphous sphericalparticle is used as a peening material having a Vickers hardness andYoung's modulus in the above ranges.

[0009] 3) One of the methods for the treatment of shot peening of theinvention is characterized in that by using the peening material (morepreferably the iron-based amorphous spherical particle) having a Vickershardness and Young's modulus in the above ranges, a substance having aVickers hardness of 950 or less (more preferably in the range of650-950) is subject to the treatment of shot peening.

[0010] 4) One of the methods for the treatment of shot peening of theinvention is characterized in that by using a peening material having aVickers hardness and Young's modulus in the above ranges, the abovesubstance is treated by peening at a speed of peening of 100 m/s or less(more preferably 50-70 m/s).

[0011] 5) One embodiment of the invention is a method of treating asubstance made of steel having a Vickers hardness of 950 or less by shotpeening, wherein the process is carried out by using an iron-basedamorphous spherical particle as a peening material, so that the treatedarticle has the characteristics of a maximum residual compressive-stressof 1600 MPa or more, and a surface roughness of 5 μm or less.

[0012] 6) A treated article of the invention is a material treated byany one of the above methods.

[0013] 7) A peening material of the invention is one having a Vickershardness in the range of 90-1100 and a Young's modulus of 200,000 MPa orless (preferably an average particle diameter of 0.02 to 1.5 mm).

[0014] 8) One of the methods of using a peening material in theinvention is one that uses a peening material having a Vickers hardnessin the range of 900 to 1100 and a Young's modulus of 200,000 MPa or lessfor the treatment of shot peening.

DETAILED EXPLANATION OF THE INVENTION

[0015] In the invention, “substance” means the material or the part madeof metal that is to be subject to the treatment of shot peening. In theinvention, a “treated article” means the substance which has beensubject to the treatment of shot peening of the invention.

[0016] As in this invention, “shot peening” is well known to one skilledin the art. It is a kind of cold-work, and means an improvement of themechanical properties of the substance.

[0017] As for the peening material used in the invention, iron-basedamorphous metals are given. Among them, an iron-based amorphousspherical particle is preferable for the peening material of theinvention because it can be manufactured relatively easily and at lowercost than in the conventional method.

[0018] The average diameter of the iron-based amorphous sphericalparticle used in the invention is not specifically limited. But, if theaverage diameter is in the range of 0.05-0.5 mm, the amorphous peeningmaterial having a Young's modulus of 50,000-150,000 can be manufacturedby the Atomize method at relatively lower cost. If the average diameterof the particle is in the range of 0.05-0.3 mm, it can be manufacturedmore easily. However, if the average diameter of the particle is from0.02 to 1.5 mm, it can still be manufactured. As the particle diameterbecomes larger, it gets increasingly difficult to obtain sphericalamorphous particles. When the diameter is greater than 1.5 mm, it isdifficult to have the shape of it be spherical. Thus, it is furtherpreferable that the average diameter of the peening material used in theinvention be 0.05 mm to 1.5 mm.

[0019] It is preferable that the content of iron in the iron-basedamorphous spherical particle used in the invention be 45 to 55 wt %.Also, the iron-based amorphous spherical particle used in the inventionmay comprise other metals. It may comprise, for example, nickel, cobalt,or the like.

[0020] As for the substance used in the invention, one having a Vickershardness of 950 or less before the treatment of shot peening ispreferable. For such hardness, by the treating process of the invention,the substance can be provided with a high residual compressive-stress,and a small surface roughness thereof can be achieved. On the otherhand, even if the Vickers hardness of the substance is 650 or less, aneffect similar to the above can be obtained. However, such a substancecan be provided with a residual stress not based on the method of theinvention, namely, by using a peening material of another type ofmaterial.

[0021] As for the substance used in the invention, carburized parts likea gear and a variety of molds, for example, fall under it. As for amaterial of it, for example, SKD, SUP, SCM, SNCM (by the JapaneseIndustrial Standards), and the like are given. Also, the shape of thesubstance is not limited, and, further, whether it is treated by heat,and the types of heat treatment, are not limited.

[0022] In this invention, the speed of peening is a concept whichincludes the speed of projection by a variety of air-operated projectionequipment as well as the speed of peening by a centrifugal peening. Thespeed of peening of 50-70 m/s is less than a normal centrifugal speed ofpeening. As for a method to carry out the treatment of peening, apressure-type and gravity-type are given. Both methods can be used inthe invention.

[0023] As examples of the other projection conditions, a projectionpressure of 0.3 MPa, a distance for the projection of 1500 mm, and aprojected amount of 4 kg/min., are given. These are representativeexamples when the invention is applied for a spring. It is to beappreciated by one skilled in the art that it is not intended to limitthe scope of the invention to these examples.

THE EFFECTS OF THE INVENTION

[0024] According to invention Nos. 1 and 2, a treated article having ahigh residual compressive-stress and a small surface roughness can beobtained, and the life cycle of such a treated article is long.

[0025] If the substance is subject to the treatment of shot peening ofinvention No. 2, the treated article has a high residualcompressive-stress, with the surface roughness thereof being controlledbelow a constant value (the softness is maintained), because theparticle as the peening material is different from the particle of thecrystal structure, and is a material with a low modulus (a low Young'smodulus). That is to say, while maintaining the softness of the surface,increases of the hardness, yield strength, and tensile strength can beachieved.

[0026] Also, according to invention No. 2, because the hardness of aniron-based amorphous particle is high, a desirable blast treatment canbe efficiently carried out.

[0027] Additionally, according to invention No. 3, when the substancehaving a Vickers hardness of 950 or less (e.g., a steel) is treated byshot peening, the treated article can be easily provided with thecharacteristics of a maximum. residual compressive-stress of 1600 MPa ormore, and a surface roughness of 5 microns or less.

[0028] Also, according to invention No. 4, even at a relatively lowspeed of peening the treated article can be provided with thecharacteristics of a maximum residual compressive-stress of 1600 MPa ormore, and a surface roughness of 5 micorns or less. Thus, invention No.4 is a method for the treatment of shot peening that contributes tosaving energy. However, by the invention the surface roughness can belowered even at a high speed of peening.

[0029] According to invention No. 5, the steel can be provided with thecharacteristics of a maximum residual compressive-stress of 1600 MPa ormore, and a surface roughness of 5 microns or less, and the life cycleof the treated article made of steel is extended.

[0030] As is evident from the above explanation, this invention is amethod for the treatment of shot peening wherein a peening materialhaving a Vickers hardness (Hv) in the range of 900-1100 and a Young'smodulus of 200,000 MPa or less is used. By this method, even if steelhaving high hardness is used as the substance, it becomes possible toprovide the substance with the property of a high residualcompressive-stress, while controlling the surface roughness thereof tobe below a constant value.

[0031] If it is desired to provide a high residual compressive-stress,generally a substance of which the hardness is higher than that of thepeening material may be selected as the substance. However, when aparticle with high hardness, such as a cemented carbide, is used, thesurface of the substance, which then becomes the treated article, willbe tremendously damaged due to the high modulus thereof. But, anamorphous shot-peening material has a high hardness and a low modulus(low Young's modulus). Thus, by the peening, treatment using theamorphous shot-peening material, the substance can be provided with ahigh residual compressive-stress, without damaging the surface thereof.

EXAMPLES

[0032] Below some embodiments of the invention are now illustrated bygiving some test examples.

Test Example 1

[0033] In Test 1, the effect in which an iron-based amorphous particlewas adopted as a peening material was studied. The iron-based amorphousparticle, of which the composition of the material is 6% cobalt, 25%nickel, 5% silicon, 3% boron, 4% molybdenum, and 57% (the remainder)iron, having a Vickers hardness of 900 to 1000, a Young's modulus of80,000 MPa, and a particle diameter of 0.3 mm, was used. The peeningmaterial used as the cemented carbide shot was made of tungsten carbide,having a Vickers hardness of 1400, a Young's modulus of 600,000 MPa, anda particle diameter of 0.3 mm. The peening material used as the caststeel shot was made of carbon steel, having a Vickers hardness of 730, aYoung's modulus of 210,000 MPa, and a particle diameter of 0.3 mm. Asthe blast equipment, “Air Blast Equipment MY30,” from Sinto Brator Ltd.,was used. As the substance, a mold made of SKD 11, the composition ofwhich is 1.5% carbon, 12% chrome, 1% molybdenum, and 85.5% (theremainder) iron, having a Vickers hardness of 770, was used. The adoptedconditions of the treatments are: the pressure of the projection: 0.4MPa; the nozzle diameter of the projection: 6 mm; the distance for theprojection: 150 mm; the amount of the projection: 80 m/min. Thetreatment of shot peening was carried out. The results are, listed inTable 1. TABLE 1 Max. residual Vickers Surface compressive-stresshardness of the roughness of the of the treated treated article treatedarticle article Ex. amorphous shot 900 2.702 1900 amorphous shot 9002.546 1850 amorphous shot 950 3.123 1850 amorphous shot 1000 3.416 1800Com. Ex. cemented carbide 1400 8.042 1950 shot cast steel shot 730 1.2611500

[0034] From Table 1, the following is understood. By the cast steel shotthat uses a peening material made of cast steel, although the surfaceroughness of the treated article was small, the treated article was notgiven a high residual compressive-stress. On the other hand, to providethe treated article with a high residual compressive-stress, peeningmaterial made of cemented carbide was used. But, the surface roughnessthereof became very high. On the contrary, “amorphous shot” that usespeening material made of iron-based amorphous was able to have aresidual compressive-stress for the treated article that is high, andthe surface roughness thereof low.

Test Example 2

[0035] Next, Test Example 2 is explained. To study the effects of thetypes of peening material and speed of peening (speed of projection) inthe peening treatment, spring material SCM420, the composition of whichis 0.2% carbon, 0.25% silicon, 0.75% manganese, 1.1% chrome, 0.25%molybdenum, and the remainder iron, having an Hv of 370, was treated.The maximum residual compressive-stress and the surface roughness of thetreated article were measured.

[0036] The amorphous peening material used in this test, the material ofwhich was the same as that used in Test Example 1, had a Vickershardness of 1000, a Young's modulus of 75,000 to 83,000 MPa, and aparticle diameter of 0.05 to 1.5 mm 0. The materials of the cast steeland cemented carbide peening materials are the same as those used inTest Example 1. The results are listed in Table 2. TABLE 2 Conditions ofprojection Result Types of peening material Residual Surface AverageYoung's Speed of compressive roughness diameter Hardness modulus peeningstress Rz (mm) (Hv) (MPa) (m/s) (MPa) (micron) Amorphous 0.05 100083,000 80 1600 2.5 [spherical 0.2 1000 80,000 80 1800 3.0 particles] 0.51000 80,000 80 1900 4.0 1.0 1000 78,000 80 1950 5.0 1.5 1000 75,000 801950 6.0 Cast steel 0.2 800 210,000 100 1100 3.0 0.3 800 210,000 1001300 5.0 0.3 800 210,000 120 1350 7.0 0.6 800 210,000 100 1200 5.0Cemented 0.06 1500 600,000 40 1900 11.0 carbide 0.2 1500 600,000 40 190013.5

[0037] It should be understood from Table 2 that when the amorphousspherical particle, i.e., the peening material that meets therequirements of the invention, was used, at a speed of peening lowerthan the one used in the case when a steel spherical shot (cast steel)was used, a high residual compressive-stress of 1600 MPa or more, whichcould not be provided when steel spherical shot was used, was able to beprovided.

[0038] Further, based on the testing, it is clear that when the aboveamorphous spherical particle is used, the treatment can be carried outwith less damage to the surface, compared to when the peening materialof cemented carbide was used to provide a high residualcompressive-stress. According to this invention, the surface roughnessof the treated article becomes about 5 microns or less, which isgenerally required. Thus, it is not necessary to carry out are-treatment such as grinding or the like.

1. A method for the treatment of shot peening characterized in that apeening material having a Vickers hardness (Hv) in the range of 900-1100and a Young's modulus of less than or equal to 200,000 MPa is used. 2.The method of claim 1, wherein the peening material has a Young'smodulus in the range of 50,000-150,000 MPa.
 3. The method of claim 1 or2, wherein. said peening material is an iron-based amorphous sphericalparticle.
 4. The method of any one of claims 1-3, wherein a substancehaving a Vickers hardness of 950 or less is subject to the treatment ofshot peening.
 5. The method of claim 4, wherein the Vickers hardness ofthe substance is in the range of 650-950.
 6. The method of any one ofclaims 1-5, wherein the treatment of shot peening is carried out at aspeed of peening of 100 m/s or less.
 7. The method of claim 6, whereinthe speed of peening is in the range of 50-70 m/s.
 8. A method oftreating a substance made of steel having a Vickers hardness of 950 orless by shot peening, wherein the method is carried out by using aniron-based amorphous spherical particle as a peening material, so that atreated article has the characteristics of a maximum residualcompressive-stress of 1600 MPa or more and a surface roughness of 5microns or less.
 9. A material treated by the method of any one ofclaims 1-8, said treated article having the characteristics of a maximumresidual compressive-stress of 1600 MPa or more and a surface roughnessof 5 microns or less.
 10. A peening material having a Vickers hardnessin the range of 900-1100 and a Young's modulus of 200,000 MPa or less.11. The peening material of claim 10, wherein an average particlediameter is 0.02 to 1.5 mm.
 12. A use of a peening material having aVickers hardness in the range of 900 to 1100 and a Young's modulus of200,000 MPa or less in a shot-peening treatment.